Blood Flow To The Lung PDF

Blood Flow to the Lung Lecture Outline I. Bronchial and Pulmonary Circulation II. Pulmonary Vascular Resistance III. Regional Distribution of Pulmonary Blood Flow: the Zones of the Lung IV. Hypoxic Pulmonary Vasoconstriction V. Effects of Mechanical Ventilation on Pulmonary Blood Flow VI. Pulmonary...

Blood Flow to the Lung Lecture Outline I. Bronchial and Pulmonary Circulation II. Pulmonary Vascular Resistance III. Regional Distribution of Pulmonary Blood Flow: the Zones of the Lung IV. Hypoxic Pulmonary Vasoconstriction V. Effects of Mechanical Ventilation on Pulmonary Blood Flow VI. Pulmonary Edema 1 Blood Flow to the Lung Objectives 1.Define the terms dead space and shunt 2.Describe bronchial circulation 3.Explain pulmonary circulation according to its functions Compare systemic and pulmonary circulation 4.Identify determinants of pulmonary vascular resistance 5.Describe the change in pulmonary vascular resistance at various lung volumes 6.Describe how the location of vessels affects their resistance 7.Describe how increased cardiac output affects pulmonary vessels 8.Understand distention and recruitment of vessels in pulmonary circulation 9.Identify the effects of gravity on pulmonary blood pressure and zones of the lung 10.Understand the mechanism and contribution of hypoxic pulmonary vasoconstriction 11.Describe hemodynamics during positive pressure breathing 12. List and describe 5 main conditions that can produce pulmonary edema 2 References Assigned reading from your text: Levitzky Chapter 4 3 I. Bronchial and Pulmonary Circulation 4 Bronchial Circulation – 2% Cardiac Output Of LV ❑ Arises from aorta or intercostal arteries and supplies tracheobronchial tree and structures to terminal bronchioles – – Drainage by azygos system of veins + a substantial portion enters pulmonary veins Part of the normal anatomic right-to-left shunt ❑ Functions - Air conditioning of inspired air – – May function as collateral circulation for gas exchange airways New vessels can branch from the bronchial veins if pulmonary circulation is blocked→ collaterals may supply other airways ❑ Regulation of bronchial circulation→ Some vasoconstriction 5 Pulmonary Circulation Is Equal To 100% Cardiac Output ❑ Entire output of the RV and supplies lung with mixed venous blood draining all tissues of the body Lung structures distal to terminal bronchioles receive O2 directly by diffusion from alveolar air and nutrients from mixed venous blood of pulmonary circulation Pulmonary circulation holds ~500 ml of blood – ~70 ml located in pulmonary capillaries – Functional pulmonary capillaries are pulmonary arterial segments and capillaries – 280 billion pulmonary capillaries supply 300 million alveoli (alveoli completely enveloped) Pulmonary capillary diameters average 6 um; average erythrocyte is ~ 8um An RBC takes 4-5 seconds to travel through at resting CO 6 Functions of Pulmonary Circulation ❑ Major function is gas exchange Also: – Blood reservoir - primarily for the left ventricle; Can transiently compensate – Pulmonary circulation holds ~500 ml of blood ~70 ml located in pulmonary capillaries – Filter to trap emboli, gas bubbles, and cellular debris etc. – Absorption of exogenous fluid from inner surface of alveoli: – Pulmonary capillary endothelium is much more permeable to water and solutes than is alveolar epithelium→ Edema fluid accumulates in interstitium before alveoli 7 Similarities To The Systemic Circulation ❑ Pulmonary circulation has: – A pump- the right ventricle – The same circulating volume/min as cardiac output from the LV → ~ 5L/min ❑ Similar vessel types: – Distributing vessels - arteries and arterioles – Exchange vessels - pulmonary capillaries – Collecting vessels - venules and veins – Normally there are four pulmonary veins that enter the left atrium One from each lobe Right upper and middle combine 8 Differences from systemic circulation ❑ Pulmonary vessels have less vascular smooth muscle and offer much less resistance to blood flow – ~ 1/10 the resistance of the systemic circulation Distribution of resistance: – 1/3 in each vessel type: distributing, exchange, and collecting – 70% of resistance of systemic circulation in distributing vessels ❑ Vessels are more compliant (more distensible) than systemic circulation – And have lower intravascular pressures – They are more compressible than systemic arteries – Pulmonary vessels in the thorax are subject to alveolar and IPPs that can change greatly. Transmural pressure difference is a major determinant of PVR – Permits for increases in blood flow without increasing blood pressure → Allows for less resistance to flow compared to systemic BP ❑ Vasoconstrict in response to hypoxia- Hypoxic Pulmonary Vasoconstriction 9 Normal pressures in pulmonary circulation ❑ Pulmonary circulation conducts the entire CO from the pulmonary artery to the elft atrium with a low driving pressure Drop in pressure in pulmonary circulation-from the PA to the left atrium is ~ 10 mmHg Drop in pressure in systemic circulation is ~ 100 mmHg 10 Pulmonary Vascular Resistance Must Be Calculated ❑ Pulmonary vascular transmural pressure difference is an important determinant of PVR – Thin walls (compared to systemic circulation) – Subject to alveolar and intrapleural pressures that can change greatly – Vascular smooth muscle can actively contract or relax in response to neural and humoral factors- but Passive factors play a major role in determining PVR – PVR is 1/10 SVR (sometimes called TPR) – Distribution of resistance: 1/3 in each vessel type: distributing, exchange, and collecting 11 II. Pulmonary Vascular Resistance 12 Lung Volume and Pulmonary Vascular Resistance ❑ Alveolar capillary walls Distend when blood volume inside increases Compress when alveolar air pressure increases ❑ At high lung volumes: Extra-alveolar vessel resistance to blood flow decreases Alveolar vessel resistance to blood flow increases ❑ At forced low volumes: Extra-alveolar vessel resistance to blood flow increases Alveolar vessel resistance to blood flow decreases 13 Location of Alveolar Vessels ❑ Location of alveolar vessels affects their resistance: Alveolar and extraalveolar vessels are two Groups of resistances in series→ Their resistances are additive at any volume ❑ PVR is usually lowest at FRC and increases at higher and lower lung volumes 14 Pulmonary Vascular Resistance Decreases with Increased Cardiac Output ❑ PVR decreases with increased CO Allows for less resistance to flow compared to systemic arteries and arterioles Effects of increased cardiac output: Increasing blood flow to the lungs (eg exercise) only increases mean pulmonary artery pressure slightly Resistance to pulmonary blood flow decreases – A passive event- not hormonal or nervous 15 Distention and Recruitment ❑ PVR decreases with increases in pulmonary blood flow, pulmonary artery pressure, left atrial pressure, or pulmonary capillary blood volume because of: – – – Distention of already open blood vessels Recruitment of previously unopened vessels Or Both Distention → increases diameter of vessels – – Increased perfusion pressure in capillaries Poiseuille’s law and r4 factor R = 8l/r4 Recruitment → opens “closed” capillaries – – Particularly in the apical region of the lung Blood flow is diverted to previously “closed” capillaries Newly opened vessels decrease resistance ❑ Both distention and recruitment occur: – – During exercise With removal of lung tissue 16 III. Regional Distribution of Pulmonary Blood Flow: the Zones of the Lung 17 Effect Of Gravity On Pulmonary Blood Pressure ❑ In physiologic person: ▪ Lungs are ~30cm in height – Pulmonary artery enters at the hilum→ two vertical columns of blood each 15cm in height One above the pulmonary artery/hilum and One below the pulmonary artery/hilum ▪ Hilar region is approximately at the midpoint, ie 15cm below apex/15 cm above base ▪ A column of pulmonary blood of 15cm exerts a pressure of 11mm hg at its base Example if pulmonary arterial pressure is 25/8: – From the pulmonary artery to apex→blood must overcome a pressure = 11mm hg 25 - 11 = 14 is pulmonary arterial blood pressure at apex 8 - 11 = -3 – From the pulmonary artery to base→ column of blood exerts a pressure of 11 mmHg→ higher 25 + 11 = 36 8 + 11 = 19 18 According to Starling’s forces→ base of the lung is the first place pulmonary edema occurs Interaction Of Gravity And Extravascular Pressure: Zones Of The Lung ❑ There is more blood flow in the lower regions of the lung than in upper regions. The effects of pulmonary artery pressure, pulmonary vein pressure, and alveolar pressure on pulmonary blood flow are described as the “zones of the lung”. Experiments done on excised lungs of experimental animals to demonstrated three zones of pulmonary blood flow Parameters: – – – PA= alveolar pressure; Pa = pulmonary arterial pressure /Systolic Even in uppermost regions is normally > alveolar pressure Pv = pulmonary venous pressure /Diastolic 19 Zones Of The Lung ❑ Zones of the Lung Zone 1- No blood flow – Capillary pressure is never > alveolar P – Ventilated but not perfused – Alveolar dead space – Does not normally exist – Hypovolemic states / PPV Zone 2- Intermittent blood flow – Normally from 10 cm above heart to apex Zone 3- Continuous blood flow – Normally from 10 cm above heart to base – During exercise and recumbent, all lung zone 3 20 Total Pulmonary Blood Flow (Cardiac Output) Thermal dilution technique with Swan-Ganz catheter Balloon tip is floated into the pulmonary artery Since no valves between the pulmonary capillaries and lumen of the LA→ PCWP is similar to LA Pressure PCWP (PAOP) estimates LVEDP or LV preload PCWP is measured with the catheter tip in Zone 3 (Pa > Pv > PA) ❑ 21 22 IV. Hypoxic Pulmonary Vasoconstriction 23 Hypoxic Pulmonary Vasoconstriction (HPV) ❑ HPV redistributes blood to ventilated alveoli- Mitigates Shunt Mitigates shunt- it is a local response to alveolar hypoxia or hypercapnia – – Diverts blood away from poorly ventilated or unventilated alveoli Not CNS- effect persists in transplant patients Arterioles supplying hypoxic alveoli constrict If PO2 of alveoli decreases, the smooth muscle in the arterioles will be exposed to decreased PO2s outside the vessel, causing them to constrict Inhibition of O2-sensitive voltage-gated K+ channels depolarizes smooth muscle cells in the small branches of the pulmonary artery The influx of calcium causes vascular smooth muscle contraction HPV occurs in response to alveolar hypoxia and alveolar hypercapnia (hypoventilation) 24 Mechanism of HPV A. Normal alveolar-capillary unit B. Perfusion of a hypoventilated alveolus sends venous blood to left atrium C. HPV increases the resistance to blood flow to the hypoventilated alveolus D. Blood is diverted away from the hypoventilated alveolus 25 Volume And Distributive Aspects Of Pulmonary Circulation ❑ There is greater blood flow per unit volume to lower regions of the lung than to upper regions of the lung This test was done at TLC Lying down- blood flow per unit volume is still greater in the gravity-dependent regions of the lung 26 V. Effects of Mechanical Ventilation on Pulmonary Blood Flow 27 Effects of Mechanical Ventilation (PPV) on Pulmonary Blood Flow ❑ During negative-pressure (eupnea) breathing During inspiration→ decreased IPP→ increased venous return Decreased blood to left heart→ decreased LV filling and stroke volume ❑ Positive pressure ventilation decreases pulmonary blood flow Decreases CO by decreasing RV preload and increasing right atrial pressure IPP is positive during inspiration without PEEP IPP is positive throughout the respiratory cycle with PEEP Also increases intrathoracic pressure which compresses great veins/ lower venous return ❑ PPV increase afterload of RV by increasing PVR During inspiration, resistance to pulmonary blood vessels increases Alveolar pressure is also positive which compresses the capillaries ❑ Positive alveolar pressures and decreased venous return Increase Zone 1 Increase alveolar dead space PPV (esp with PEEP) may decrease or prevent low ventilation-perfusion ratios (shunt) 28 Type Phase Eupnea Inspiration Eupnea Expiration PPV Inspiration PPV Expiration IPP RV Filling & SV RVCO Total thoracic blood volume LV Filling & SV LVCO 29 VI. Pulmonary Edema 30 Pulmonary Edema – Extravascular Accumulation Of Fluid In The Lung ❑ Starling’s principle of capillary exchange- normal conditions – Net reabsorption of water is into the pulmonary capillary at arteriolar and venule ends – This net reabsorption is one mechanism for absorbing exogenous water from alveoli Plasma colloid osmotic pressure- does not change along the length of capillary – The only force that can be measured clinically Hydrostatic pressure of plasma along capillary- remains less than PC of plasma 31 Factors Predisposing To Pulmonary Edema: Starling Forces and Surface Tension ❑ Capillary endothelium is more permeable to water and solutes than alveolar epithelium In pulmonary edema: Fluid moves from capillaries → interstitium → alveolus 1. Factors associated with Starling’s principle: – Increased alveolar-capillary unit permeability allows fluid to flow → alveolus Infection, O2 toxicity, circulating or inhaled toxins – Increased pulmonary hydrostatic pressure (pc) - Pulmonary hypertension Fluid overload from excess IV fluids CHF, mitral stenosis, left ventricular MI, emphysema – Decreased plasma colloid osmotic pressure (pc) – Protein loss Renal and liver disease and malnutrition 2. Surface tension: – – In surfactant deficiency, surface tension forces are increased Fluid is drawn from pulmonary capillary to inner surface of alveolus 32 Factors Predispose To Pulmonary Edema: A Very Negative IPP 3. A very negative IPP can lead to pulmonary edema – Forceful inspiration against a closed glottis or other upper airway obstruction creates Laryngospasm can result in negative-pressure pulmonary edema: – – – – Laryngospasm is an airway obstruction- the spasm is maintained after stimulus passed – – – Morbidity/mortality related immediately to hypoxemia and hypercapnia Obstructions not relieved immediately→ negative-pressure pulmonary edema Deepen anesthetic, Larson’s maneuver, give succinylcholine Clinical picture related to degree of obstruction and inspiratory effort: Partial airway obstruction produces high-pitched stridor – A very negative IPP is created to overcome resistance for inhalation→ Reduces the interstitial hydrostatic pressure → creates a subatmospheric alveolar P → Promotes transudate of fluid from capillaries→ interstitium→ alveoli Small vessel damage may result in frank hemorrhage into alveoli (pink froth) Give positive pressure ventilation with a facemask, head extension, and jaw thrust Complete obstruction is silent – Positive pressure mask ventilation forces air into piriform fossa 33 Factors Predispose To Pulmonary Edema: Pulmonary Hypertension 4. Pulmonary hypertension can lead to pulmonary edema Causes of pulmonary hypertension: – Primary or idiopathic pulmonary hypertension – Left heart failure – Emphysema Destruction of alveolar septa/pulmonary capillaries H+ ions from acidosis causes vasoconstriction – Hypoxia causes vasoconstriction in pulmonary vessels → HPV 34 Factors Predispose To Pulmonary Edema: Hypoxic Pulmonary Vasoconstriction 5. Hypoxic pulmonary vasoconstriction Alveolar hypoxia or atelectasis → active vasoconstriction in pulmonary circulation Vascular smooth m. constriction→ near the alveoli in the arterial, precapillary vessels May be localized to small region of lung or global Global hypoxia of the whole lung→ eg at high altitude → pulmonary edema 35 1. Which of the following situations would be expected to decrease pulmonary vascular resistance? A. Ascent to 15,000 ft above sea level B. Inspiration to the total lung capacity C. Expiration to the residual volume D. Moderate exercise E. Blood loss secondary to trauma 4–3. 2. Which of the following situations would be expected to lead to an increase amount of the lung under zone 1 conditions? A. Ascent to 15,000 ft above sea level B. Blood loss secondary to trauma C. Moderate exercise D. Positive-pressure ventilation with positive end-expiratory pressure (PEEP) E. Changing from the standing to the supine position 3. Which of the following circumstances might be expected to contribute to the formation of pulmonary edema? A. Overtransfusion with saline B. Occlusion of the lymphatic drainage of an area of the lung C. Left ventricular failure D. Low concentration of plasma proteins E. Destruction of portions of the pulmonary capillary endothelium by toxins F. All of the above 36